It is known that polymers can be made electrically conductive by dispersing therein suitable amounts of conductive particulate fillers such as carbon black or fine metal particles. Over recent years, there has been particular interest in such compositions that exhibit positive temperature coefficient (PTC) characteristics, i.e., which show a very rapid increase in resistivity over a particular temperature range.
PTC materials are conductive materials characterized by a sharp increase in resistivity upon reaching a switching temperature (Ts). If the jump in resistivity is sufficiently high, the resistivity effectively blocks the current and further heating of the material such that overheating of the material is prevented. One of the main benefits of PTC materials is that no additional electronic circuits are necessary in an article that includes a PTC material since the PTC material itself has a characteristic similar to electronic circuits. Moreover, upon cooling, the material resets itself. This jump in resistivity may oftentimes be referred to as the PTC amplitude and may be defined as the ratio of the maximum volume resistivity to the volume resistivity at room temperature (app. 23° C.).
FIG. 1 is an exemplary depiction of the behavior of an electrically conducting polymeric PTC composition when subjected to a change in temperature. FIG. 1 depicts the changes in electrical resistivity when the material is subjected to a change in temperature. The electrical resistivity increases from the bottom to the top of the y-axis. As can be seen at the switching temperature Ts, there is a change in resistivity of several orders of magnitude. The switching temperature is indicated by the letter (A) in FIG. 1. At temperatures lower than the switching temperature, the resistivity of the polymeric composition does not change very much with a change in temperature (region 1). The slope of the resistivity curve prior to the switching temperature is referred to in FIG. 1 as the first slope. At temperatures greater than the switching temperature, there is an increase in the electrical resistivity of several orders of magnitude with temperature (region 2). This is referred to as the second slope. The switching temperature (A) is defined as the intersection of a tangent taken to the first slope with a tangent taken to the second slope. After the resistivity has increased rapidly with temperature, a maximum in resistivity is reached. The PTC amplitude is often defined as the ratio of the maximum volume resistivity to the volume resistivity at room temperature (app. 23° C.). Another way to express the PTC effect is defining the volume resistivity at a certain temperature X divided by the room temperature resistivity (RX° C./R23° C.). As the temperature is raised further, however, often the electrical resistivity of the polymeric composition drops with temperature (region 3). The electrically conducting polymer composition now displays a negative temperature coefficient of resistance (NTC). This change from PTC behavior to a strong NTC behavior is often undesirable. There have been several theories put forth to explain both the PTC and the NTC effect. Polymeric PTC composites are based on semi-crystalline polymers and conducting fillers whose concentration is just above the percolation threshold. The mechanism for the PTC anomaly in semi-crystalline polymers is attributed to the relatively large change in specific volume of the polymer at the onset of melting. Due to this volume expansion stresses are generated resulting in displacement of part of the conductive fillers thereby disrupting the conductive chains. In addition and due to increased amorphous volume, the concentration of conductive particles effectively decreases. Both factors result in a sharp increase in resistivity. With increasing temperature the mobility of the conductive fillers increase, resulting in a reconnection of the conductive fillers opposing the initial contact loss and PTC effect and leading to a Negative Temperature Coefficient (NTC) effect on the resistivity.
PTC materials have been utilized in self-controlled heaters. When connected to a power source, the PTC material will heat up to the trip temperature and maintain this temperature without the use of any additional electronic controllers.
Compositions exhibiting PTC behavior have also been used in electrical devices as over-current protection in electrical circuits comprising a power source and additional electrical components in series. Under normal operating conditions in the electrical circuit, the resistance of the load and the PTC device is such that relatively little current flows through the PTC device. Thus, the temperature of the device remains below the critical or trip temperature. If the load is short circuited or the circuit experiences a power surge, the current flowing through the PTC device increases greatly. At this point, a great deal of power is dissipated in the PTC device. This power dissipation only occurs for a short period of time (fraction of a second), however, because the power dissipation will raise the temperature of the PTC device to a value where the resistance of the PTC device has become so high, that the current is limited to a negligible value. The device is said to be in its “tripped” state. The negligible or trickle through current that flows through the circuit will not damage the electrical components which are connected in series with the PTC device. Thus, the PTC device acts as a form of a fuse, reducing the current flow through the short circuit load to a safe, low value when the PTC device is heated to its critical temperature range. Upon interrupting the current in the circuit, or removing the condition responsible for the short circuit (or power surge), the PTC device will cool down below its critical temperature to its normal operating, low resistance state. The effect is a resettable, electrical circuit protection device.
Various materials have been developed that show these characteristics. Among them are ceramics as well as polymer based PTC materials. One problem is that most PTC materials exhibit Negative Temperature Coefficient (NTC) characteristics immediately after the PTC characteristics. If the jump in resistivity is not sufficiently high or if the NTC effect is pronounced it means that current can start to flow again. This is unwanted and efforts have been undertaken to reduce or eliminate the NTC effect.
In polymeric PTC materials reduction of the NTC effect has been achieved by cross-linking the material. Most effective is post-crosslinking after the forming step either by gamma radiation or accelerated electrons. Cross-linking in the melt also erases the NTC effect but negatively affects the PTC amplitude. In addition, the step of cross-linking the material increases the time and production costs for manufacturing the PTC material.
Accordingly, it would be beneficial to provide a PTC material that has a reduced negative temperature coefficient effect as compared to prior art PTC materials to help reduce some of the disadvantages associated with prior art materials. It would also be beneficial to provide a PTC material that has a reduced negative temperature coefficient effect other than by cross-linking of the material.